Fluoropolymers, in particular tetrafluoroethylene [TFE]-perfluoro(alkyl vinyl ether) [PAVE] copolymers (hereinafter referred to as “PFAs”), TFE-hexafluoropropylene [HFP] copolymers (hereinafter referred to as “FEPs”) and like TFE copolymers, are well-known melt-moldable fluororesins and are widely used as rotational molding materials in manufacturing tubes or pipes, joints, containers or vessels, wire coverings and like moldings, or coatings, linings or hollow moldings.
Due to the mechanisms of polymerization thereof, fluoropolymers generally have one or more thermally unstable terminal groups other than —CF3, for example —COF, —COOH, —CH2OH and —COOCH3, depending on the polymerization initiator species and chain transfer agent species, among others, used on the occasion of polymerization thereof. The occurrence of such terminal groups in fluoropolymers may cause blowing or hydrofluoric acid generation during molding, which in turn may lead to failure in molding or mold corrosion. Further, in the semiconductor field in which the demand for cleanliness is increasing year by year with the increasing scale of integration of devices, it is strongly desired that these unstable terminal groups be stabilized.
Patent Document 1 (Japanese Kokai Publication S62-104822) describes a method of overcoming this problem which comprises bringing a specific TFE-PAVE copolymer into contact with a fluorine-containing gas under temperature, time and pressure conditions sufficient for removing all unstable terminal groups and further blowing an inert gas against the copolymer to thereby extract the unstable terminal groups therefrom. However, this method requires a long period of contacting with fluorine gas at an elevated temperature and, therefore, it is feared that the contamination of the copolymer as resulting from apparatus corrosion may progress and/or the degradation of the copolymer may affect the physical properties thereof.
On the contrary, in Patent Document 2 (Japanese Kokai Publication H04-20507), there is describes a method which comprises bringing a specific TFE-PAVE copolymer into contact with fluorine gas to attain a total number of —COF and —COOH groups of 7 to 40 per 106 carbon atoms and then further with ammonia gas to convert the —COF groups totally to —CONH2 groups.
On the other hand, in Patent Document 3 (Japanese Kohyo Publication (laid-open Under PCT) H03-500547), a method of stabilizing fluoropolymers is described which comprises bringing a —COF— and —COOH-containing copolymer into contact with an organic amine or a tertiary alcohol to give an intermediate and, after drying the same, fluorinating the same to accomplish the fluorination at lower temperatures as compared with the conventional methods of —COF fluorination.
However, this method has the problem of byproduct —COF formation during intermediate fluorination as well and, in the case of a copolymer having a terminal group low in reactivity with the organic amine or tertiary alcohol, for example —CH2OH or —COOCH3, the problem that the copolymer will not be converted to the desired intermediate but the terminal group will remain unreacted.
Further, Patent Document 4 (Japanese Kokai Publication H06-279603) describes a method of fluorinating the surface of a carbonaceous material-containing molded article with a reactant gas comprising a fluorine atom-containing compound after preliminary oxidation of the surface in the manner of gaseous phase oxidation, liquid phase oxidation or electrolytic oxidation. In particular, —COF can be converted to —COOH with ease by treatment with steam and the above method can be said to be effective and economical as compared with other methods of oxidation; however, in the case of a copolymer having an unstable group(s) other than —COOH or —COF, the method is not always effective. This method is nothing but a molding surface treatment method and no mention is made of carbonaceous material-free fluoropolymers. Further, in the case of copolymers having a terminal group difficult to fluorinate, no effective oxidation treatment method is available and the terminal group remains unreacted, causing the problem of insufficient stabilization.